Digital position control systems comprise a digital compensator that regulates position of a mechanical plant or actuator. The compensator responds to periodically sampled position data from a transducer and data converter that respectively measures and discretizes plant position or relative position. The compensator provides a sequence of control values that, when converted and amplified, constitute a control signal such as a motor current command signal which is provided to the actuator. The control signal attempts to correct the position of the actuator to that commanded and to decrease position error caused by disturbances in the system.
Fixed servo control compensators are optimized for a particular plant condition to achieve a specified loop gain and bandwidth. In practical servo position control systems, plant gain may vary substantially due to changes in motor torque constant in plants that use rotary actuators, or motor force constant in plants that use voice coil-type actuators, power amplifier gain, payload inertia, position sensor sensitivity, or other gain variations in the control electronics. In servo systems for head positioning in disc and tape drives used for information storage and the like, variations in overall plant gain are also often due to variations in track position detector gain, data head gap width and tolerances in the controller electronics.
Such overall plant gain variation directly affects the overall gain of the servo loop and, in so doing, affects servo bandwidth. The servo compensator, which is designed and optimized for a particular servo loop bandwidth, operates the servo loop suboptimally when bandwidth variations occur. Changes in loop gain may reduce the disturbance rejection properties of the closed loop control system, may act to make the system sluggish or, conversely, to destabilize the control system, especially when mechanical resonances are present. If high levels of performance are demanded of the compensator design, the compensator may stabilize the control loop only over a narrow range of loop gain and bandwidth, and system performance usually degrades rapidly when minor deviations in loop gain from the nominal are present.
As part of the design process itself, designers have used frequency response measurement equipment to identify the response of an open loop servo system and to manually adjust the gain and dynamics of the compensator to achieve a target bandwidth. This requires the injection of signals (typically sinusoidal or random noise) into the servo loop.
Measurement instruments typically use synchronous demodulation or Fourier transform (FFT) techniques to determine the gain at the frequency of the desired bandwidth. These, however, are manual, non-adaptive techniques that result in a one-time gain adjustment to the servo loop. Using this measured data, a fixed compensator can then be determined which will perform optimally, but only under the plant condition from which the data was obtained. If this condition changes either over time or from one system to another (in the case where the same compensator design is used for different mechanical plants) the fixed compensator is incapable of adapting and suboptimal performance results.
Therefore, it is desirable to quickly and accurately determine the overall plant gain in real time and adjust the open loop gain so that the total open loop unity gain bandwidth remains constant. Adaptive control is desirable to account for overall plant gain variation including that caused by actuator motor torque constant, payload inertia and sensor gain, whose combined variations may otherwise result in unacceptably large changes in overall plant gain. This gain variation may act to destabilize the servo system or reduce its performance.
Adaptive techniques disclosed in the prior art generally require the acquisition and analysis of a relatively very large set of data samples and consequently a lengthy batch computation time. Accordingly, such methods are unable to quickly and accurately estimate the plant gain and provide proper loop gain compensation. These methods are also sensitive to broadband noise contributed by data converter quantization, high frequency resonance activity in the mechanical plant, low frequency disturbances such as track runout and bias forces and other DC disturbances known to disturb the servo loop in disc drive systems.